This Homemade Chronograph is partially built on Jim's homemade chronograph design (body) but we re did all the circuits for more consistant and accurate IR readings. This is Jim's.

I have included our circuit schematics, which work amazingly well.As you will see we did included IR LEDs. This along with the capacitor helps amplify the signature. There are also a 10k resistor on the phototrasitor circuit and 2 180k resistors on the IR LED circuit.

Kinda a rough drawing but its ok

For the body we cut a piece of 1.5" pipe lengthwise, slim on one side and thicker on the other. then we cut pieces out of the thicker part to match the bottom. These are the holders for the IR LEDs.

Then we drilled 7/32" holes for the "light holes" and each LED hole which was 5mm or about 13/64" bit. These are attached by electric tape because I ran out of hose clamps, but soon to be replaced. The Phototransitor hubs are 3/4 pipe and caps but not glued together. This is for servicing if needed.

We use audacity for now and just get the distance between the two drops and divide 1 by it to get our FPS.

Sorry I missed this one somehow. Nice job setting it up. One quick question...without checking into the current specs for the IR LEDs, off the top of my head, the 180k resistors seem a little high valued for a 3 volt main supply. Again, just shootin' from the hip here. Are those LEDs especially low current draw?

However, obviously it's working for you, congratulations. I may have to whip something up similar...a little larger so it can accommodate golf balls.

BTW, are you sure the resistors aren't 180 Ohms and not 180K Ohms? LEDs are usually rated at something like 1.4V 25mA. Using the 3V supply means the resistor needs to drop 1.6V at 25mA. V=IR, V/I=R=1.6V/0.025A=64 Ohms. At 180,000 Ohms the LED would only have ~9 microamps going through it. I don't think an IR LED will light with that little current. If the thing is working with a 180K resistor then it is reading ambient light since the LEDs aren't lighting up at all.

On the drawing what is the "2V" that looks like a capacitor between the phototransistors and the plug?

I've already alerted him to the resistor problem. I would be curious if he just temporarily disconnected the LEDs if it would continue to operate from ambient light as you suggested.

Unlike an incandescent bulb, an LED will still emit small numbers of photons with even a small current. The question however, is it enough for the phototransistors to detect it in a meaningful way in this application...I'm guess not....but never know until you try it.